Generally, the manufacture of polytetramethylene terephthalate by the direct polymerization of terephthalic acid and 1,4-butanediol comprises a first stage reaction in which bis(4-hydroxybutyl) terephthalate and its low polymers are prepared by subjecting terephthalic acid and 1,4-butanediol to an esterification reaction, like the manufacturing method based on an ester-interchange process between dimethyl terephthalate and 1,4-butanediol as the starting materials, and a second stage reaction in which the product of the first stage is subjected to a condensation polymerization.
In the first stage esterification reaction of this direct polymerization process, the reaction will be notably delayed or will not advance at all beyond a certain stage, if the molar ratio of terephthalic acid to 1,4-butanediol and the reaction temperature are not specified within the specific ranges even in the presence of a catalyst.
Thus for the esterification reaction to be accomplished, the molar ratio of terephthalic acid/1,4-butanediol should preferably fall in the range of 1/1.6.about.1/2.0 and the reaction temperature in the range of 210.degree..about.230.degree. C. Further, addition of a catalyst such as a titanium compound or a tin compound, is necessary.
However, an esterification reaction, even if the aforementioned conditions are met, involves problems in that 1,4-butanediol is readily dehydrated to close its ring owing to the acidity of terephthalic acid and heat, yielding tetrahydrofuran as a byproduct and giving rise to loss of 1,4-butanediol and the need for an extended period of time before completing the reaction.
Under these circumstances, the present inventors have made every effort in studies aimed at slashing the polymer manufacturing cost by acceleration of the esterification reaction, while inhibiting the formation of tetrahydrofuran as a byproduct, in the manufacture of polytetramethylene terephthalate by the direct polymerization process, and have found that notable curtailment of the esterification time as well as appreciable inhibition of the formation of tetrahydrofuran as a byproduct may be achieved by addition of an acid amide compound as the esterification reaction accelerator, which finding has led to this invention.